The system shown in FIG. 1 illustrates the essential components of the optical portion of a projection display system having three reflective spatial light modulators in the form of liquid crystal display (LCD) panels, also referred to as liquid crystal light valves (LCLV). The prior art system, depicted generally at 10 includes a light source 12, an illumination mechanism for collecting the light and concentrating it onto the light valves, shown generally at 14, a polarizing mechanism for polarizing the light, if the light valves modulate via polarization effects, shown generally at 16, a splitting mechanism for splitting the illumination into three color bands to separately illuminate the three light valves, shown generally at 18, a recombining mechanism for recombining the three light distributions after reflecting from the light valves, shown generally at 20, and a projection mechanism for projecting the combined images onto a viewing screen, shown generally at 22.
Lamp 24 and lamp reflector 26 produce and concentrate the light for this system. A series of dichroic filters 28, 30 is used to split the light from the lamp into separate red, green, and blue components. The light in each of the three components, or channels, is then polarized with a polarizing beam splitter (PBS) 32, 34, 36, and illuminates three separate LCDs, 38, 40, 42. The LCDs selectively modify the polarization of the light reflected from them allowing some portion of the light to pass back through the PBS. A second series of dichroic filters, 44, 46, is used to recombine the modulated light distributions and pass them on to a projection lens 48 imaging all three LCDs onto the viewing screen.
The configuration shown in FIG. 1 is functional and has been used to implement projection display system products. However, the large number of components in this architecture is cumbersome, and necessitates a relatively large physical size of the system. The most serious drawback to these systems is the requirement of a large back working distance for the projection lens.
A single filter, or PBS plate, tilted at 45 degrees requires an optical path length equal to or greater than the active width of the LCD panel. It may be seen in FIG. 1 that two of the three channels, green and red, require a PBS and two dichroic filters. These channels require a minimum optical path length between the LCD and the projection lens of three times the active width of the LCD. The blue channel in FIG. 1 requires only one PBS and a single dichroic filter, but the path length must be equal to the other two channels for in-focus registration of all three images on the viewing screen. The actual optical path length for the projection lens must also account for the divergence of the light after reflecting off the LCD panel. This is a function of how fast the optical system is running, usually specified by the f/# of the optical system. The minimum distance referred to here is strictly valid only for systems of very high f/# and thus impractical due to low light throughput. However, for comparison with other systems this minimum figure is a good baseline. The only advantages of this architecture is the ability to optimize the color filtering with the interaction of multiple dichroic structures and the ability to optimize the PBS performance for the narrow band color channels. However, these advantages are relatively minor.
The most straightforward method of simplifying the projector architecture is to have the filter and beam splitter structures perform more than one function in the set of required system operations. System configuration 50, shown in FIG. 2, incorporates two of these simplifications. The first is the use of a single PBS 52 immediately after lamp 24, replacing the three PBS plates of the FIG. 1 system configuration. Single PBS 52 polarizes the broadband output of the lamp prior to the color splitting operation and thus functions as the amplitude modulation control mechanism for all three LCDs. This requires that the PBS function over the entire visible spectrum. The second simplification is to utilize the same set of dichroics to split the light into the three color channels and to recombine the reflected light prior to the projection optic. This requires that the dichroic filter passbands be carefully controlled since there are now only two filters 54, 56, to control the whole system colorimetry. The savings in system complexity is readily evident.
One system difficulty not addressed by the configuration in FIG. 2 is the reduction in the back working distance of the projection lens. The projection lens must still work over a distance that is a minimum of three times the active width of the LCD. A solution to this problem is found by recognizing that the operation of the dichroic filters is still the same even if the dichroic structures are crossed as shown in system 60 in FIG. 3a. This allows the back working distance to be reduced by 33% over the systems of FIGS. 1 or 2, to a minimum of twice the active width of the LCD. Unfortunately, crossing plate dichroics 62, 64, introduces a problem because the operation at the intersection of the two plates is usually disrupted by the thickness of the plates, producing a seam in the middle of the image, where the images of the three LCD panels are totally or partially obscured by the plate intersection.
The preceding problem is solved in system 70 of FIG. 3b by the introduction of a four piece color cube filter, shown generally at 72. Dichroic filters 74, 76 are deposited on the surfaces of the four cube segments and the pieces are then glued together to form a solid cube with the dichroics sealed in the interior across the cube diagonals. If properly assembled this arrangement eliminates most of the obstruction of the central crossover of the two dichroic layers. However, this assembly is precise and the color cube component is expensive because of the difficulty in assembly. FIG. 3b also shows the use of a polarizing beam splitter cube 78. This component is a common assembly for optical systems and is not an expensive addition due to the significantly less stringent assembly requirements.
As indicated in these system configurations, the state-of-the-art in system architectures for reflective LCDs includes several arrangements, each with particular advantages and disadvantages. A desired alternative is a system that has the small back working distance advantages of the systems shown in FIGS. 3a and 3b, without the costly addition of a precisely assembled crossed dichroic filter cube.
U.S. Pat. No. 4,127,322, to Jacobson et al., Nov. 28, 1978, is one of the oldest patents found covering any type of projection display and is fashioned around the optically addressed Hughes liquid crystal light valve (LCLV). A lamp output is polarized by a beam splitter and then divided into the three color paths by dichroic filters. This configuration is equivalent to the system in FIG. 2 of the prior art. The reference also includes an alternative embodiment in which an additional set of dichroic filters and three light valves are arranged to use the light normally discarded by the polarizer. This attempt to recover the unused portion of the light is intended to improve system throughput.
U.S. Pat. No. 4,650,286, to Koda et al., Mar. 17, 1987, and U.S. Pat. No. 4,836,649, to Ledebuhr, et al., Jun. 6, 1989, describe architectures for the reflective LCLVs that are essentially equivalent to the system of FIG. 1, with the exception that they use a separate projection lens for each of the three light valves.
U.S. Pat. No. 5,239,322, to Takanashi et al., Aug. 24, 1993, is another system designed originally for the optically addressed LCLV type light modulators. The system covered in this patent is easily be recognized as equivalent to the prior art architecture of FIG. 3b. In this system the LCLVs are illuminated with images indicated as write light distributions. CRTs are typically used to produce these write light distributions and are usually abutted directly to the corresponding light modulator.
U.S. Pat. No. 5,648,860, to Ooi et al., July 15, 1997, uses two dichroic plates to separate the light into the three color channels and to recombine the light reflected from the LCDs. The angles of the plates used in this configuration are not 45 degrees and are set to try to reduce the back working distance for the projection optics. The key invention of this patent appears to be the use of positive lens elements directly in contact with the LCD panels to collimate the incoming illumination and to converge the reflected light, and the use of "cone-like" prisms to affect the matched convergence of the illuminating light. In all other aspects this system is essentially the same as that of FIG. 2.
U.S. Pat. No. 5,658,060, to Dove, is a system having a set of external dichroic filters that separate the light into the three color paths. The light in each path is separately polarized before illuminating the light valves. The reflected light is recombined through a special prism arrangement, usually referred to as a Philips prism. The Philips prism is used to try to reduce the back working distance requirements of the projection lens. Although this system uses a prism for recombination, it is architecturally equivalent to the system in FIG. 1. The reference also describes another embodiment that uses a cube beam splitter for recombining the light output but continues to use separate dichroics to affect the initial split of the light into three color paths and to use separate PBSs for each light valve.
U.S. Pat. No. 5,621,486, to Doany et al., Apr. 15, 1997, describes a simple configuration for a three-panel projector. The system uses a Philips type prism to split the illuminating light and to recombine the reflections from the three LCDs. However, this setup uses a single cube polarizing beam splitter in front of the color splitting prism. This system is thus equivalent to the architecture of FIG. 3.
U.S. Pat. No. 5,233,385, to Sampsell, Aug. 3, 1993 and U.S. Pat. No. 5,612,753, to Poradish, et al., Mar. 18, 1997, describe projection systems designed for the TI digital micro-mirror device (DMD) light modulator. These references include system architectures for both single panel color field sequential systems and multiple panel systems. In the multiple panel systems that might be compared with the present invention a color splitting prism of the Philips type is used to perform the color separation and recombination and a total internal reflecting (TIR) prism is used to get light on and off of the DMDs. In this case the system looks essentially the same as that of FIG. 3b with a TIR prism used in place of the PBS cube.
References describing transmissive light modulators include U.S. Pat. No. 5,321,448, to Ogawa, Jun. 14, 1994, and U.S. Pat. No. 5,626,409, to Nakayama et al., May 6, 1997, the latter of which describes a system which is the transmissive equivalent of the system in FIG. 1. The light from the lamp is divided into the three-color paths by a set of dichroic filters. After passing through the three light valves, the modulated light distributions are recombined using a separate set of dichroic filters. The '448 reference uses a set of dichroic filters to divide the lamp output into the three-color paths, and a separate set of dichroic filters, in the form of a color cube prism, to recombine the modulated light. This later configuration is the most common architecture presently used for transmissive light valves.